sql - Touch function in Postgres with PostGis -

i have postgresql installed postgis, try run following:

select n1.edname "borders royal exchange a" eds_census2011 n1, eds_census2011 n2 touch(n1.the_geom, n2.the_geom) , n2 = 'royal exchange a' 

and error (below) there have add postgres or enable something?

error:  function touch(geometry, geometry) not exist line 3: touch(n1.the_geom, n2.the_geom)               ^ hint:  no function matches given name , argument types. might need add explicit type casts.  ********** error **********  error: function touch(geometry, geometry) not exist sql state: 42883 hint: no function matches given name , argument types. might need add explicit type casts. character: 96 

run these 2 queries (in same session trying above query) pin down problem:

in schema function touch() live?

select p.proname, n.nspname   pg_proc p join   pg_namespace n on n.oid = p.pronamespace  proname = 'touch'; 

what current schema search_path of role:

show search_path; 

if function exists, schema must in current search_path postgres can find it. how adapt search_path?
how search_path influence identifier resolution , "current schema"

btw, can't find function touch() in function reference of postgis manual. there 1 called st_touches(). chance meant one?

be aware query has cost of o(n²), since calculates value every combination of 2 qualifying rows in eds_census2011. if condition n2.edname = 'royal exchange a' selective enough, won't problem.

also, may want exclude rows joining additional where item like:

and n1.pk_id <> n2.pk_id  

error after update

your updated query makes more sense:

select n1.edname "borders royal exchange a"   eds_census2011 n1, eds_census2011 n2  st_touches(n1.the_geom, n2.the_geom)=1 ,    n2.edname = 'royal exchange a'; 

but st_touches() returns boolean, clause should be:

where  st_touches(n1.the_geom, n2.the_geom)